To the ancient Greeks and Persians, the lake's immense size suggested it was an ocean, hence its name. The northern part of the lake is just 5 to 6 m (16 to 20 ft) deep. The southern end, however, plunges more than 1,000 m (3,300 ft). Just as the lake reaches a greater depth in the south, the nearby land reaches a greater height.

Location of the Caspian Sea in Central Asia. Source: Demis

Regional setting of the Caspian Sea. Source: K. musser

The mountains of northern Iran line the southern end of the giant lake. In marked contrast to the mountains, sand seas line the southeastern and northern perimeters of the lake, and marshes occur along the lake shores in Azerbaijan to the west.

Multiple rivers empty into the Caspian Sea, the Volga being the largest.

Lacking an outlet, the Caspian Sea loses water only by evaporation, leading to the accumulation of salt. Although a lake, the Caspian is not a freshwater lake; the water delivered by the Volga River minimizes the lake's salt content at the northern end, but the Caspian grows more saline to the south. Kara-Bogaz-Gol is a saline inlet along the lake's eastern perimeter.

The volume of this endorheic basin is roughly 78,200 cubic kilometers.

This water body became landlocked approximately five and one half million years ago, due to tectonic uplift and subsiding ocean levels. Since its origin was as a part of the ancient Paratethys Sea, the Caspian Sea has a pronounced salinity, although it is only about one third the saline content of the world oceans due to isolation and freshwater influx.

Geography

The Caspian Sea is the largest surface body of water on Earth (as measured in surface area) that is surrounded by land; in fact, the volume of water in the Caspian Sea is approximately two fifths of the total world lake water volume.

The Caspian Sea can be considered as comprised of three distinct physical regions: the northern, middle and southern Caspian. The transition between north and middle sections is known as the Mangyshlak Threshold, which extends through Chechen Island and Cape Tiub-Karagan. The dividing line between the middle and south sections is termed the Apsheron Threshold, a sill of tectonic origin[1] that runs through Zhiloi Island and Cape Kuuli.[2] Garabogazköl Bay is a saline eastern inlet of the Caspian, which is part of Turkmenistan and in earlier times has been a separate lake being divided by an isthmus which divides it from the Caspian itself.

Divisions between the three regions are dramatic. The northern reach consists of an extremely shallow zone known as the Caspian shelf,[3] .This shelf amounts to less than one percent of the water volume of the total Caspian with an average depth of only about five metres. The water depth increases substantially in the mddle Caspian, where the mean depth is 190 m and the water volume is about one third of the total Caspian Sea. The southern Caspian is deeper yet, reaching a maximum depth exceeding one kilometer; the southern Caspian comprises almost two thirds of the sea's volume. The northern portion characteristically freezes over every winter, but only in the most severe winter will the southern reach experience even a thin ice surface.

Inflows

There are more than 130 rivers or streams that discharge to the Caspian Sea, the Volga River being the highest influx. A second affluent, the Ural River, flows in from the north, and the Kura River flows into the sea from the west.

In earlier times the Amu Darya River of Central Asia in the east frequently changed course to discharge into the Caspian through a now-desiccated riverbed called the Uzboy River, as in the case of the Syr Darya farther to the north. The Caspian holds several small islands chiefly situated in the north and have a collective land area of approximately 2000 square kilometres.

Adjacent to the north Caspian is the Caspian Depression, a very low-lying area approximately 27 metres below mean sea level. The Kazakh forest steppe extends across the northeast coast, while the greater Caucasus mountains line the western coastline. The biomes to both the north and east can be described as cold, continental deserts. Conversely, the climate to the southwest and south is warm with uneven elevation due to a mix of highlands and mountain ranges; the dramatic changes in climate alongside the Caspian have led to considerable biodiversity in the region.

A large expanse of clear sky permitted this natural-color satellite image of the entire water body. The color of the Caspian Sea darkens from north to south, thanks to changes in depth and perhaps sediment and other runoff. The mountains of northern Iran line the southern end of the giant lake, and emerald green vegetation clings to those mountain slopes. Image courtesy of NASA.

Baku is Azerbaijan's major city, and the oil capital of the Caspian region. This photo shows details of the city, including the extensive port facilities, and part of the large web of offshore oil platforms in the Caspian Sea. The oil platforms off Baku were built in the 1950s and 1960s, and were the first offshore oil-drilling efforts in the world. Today, multinational oil exploration, sea-level rise (the Caspian Sea has risen more than 2 m in the past 20 years), offshore platform maintenance, and environmental degradation are all hot topics in Baku. Image courtesy of NASA.

Ecology

The Caspian Sea has important ecological resources not only in its waters, but also on its numerous islands and vast coastal areas. Within the sea itself is a sizable sturgeon population; this species is well known due to the harvest of sturgeon roe that are processed into Caspian Sea caviar. Overfishing of sturgeon has reduced the sturgeon fishery, such that many have called for a ban on the taking of sturgeon. Although some regulations have been adopted to protect sturgeon stocks, the lucrative caviar market renders enforcement of these statutes problematic. Caviar harvesting is particularly destructive to the sturgeon population, since the practise targets reproductive females.

The Caspian seal (Phoca caspica), is an endemic speciesto the Caspian Sea, and is one of the few pinnipeds to occur in inland waters (The Baikal seal and Saimaa Ringed seal also share this characteristic). The Caspian Sea lends its name to several species of birds, including the Caspian gull and the Caspian tern. Most of the fish species within the Caspian are freshwater species, who are adapted to the brackish conditions; Greater pipefish, Hardyheads and two mullet species are the only chief true sea species found in the Caspian. There are several fish taxa endemic to the Caspian, including the Caspian white fish, Caspian Rutilus, Caspian bream (There are certain sightings of the latter species within the Aral Sea.), and a Caspian salmonoid, the Caspian salmon, Salmo trutta caspiensis, which is classified as critically endangered.

There are a number of alien aquatic species that have been introduced into the Caspian Sea from other world regions that have been acknowledged as invasive in status; these include the New Zealand mud snail, Potamopyrgus antipodium, and the Chinese mitten crab, Eriocheir sinensis.[4] Dumont et al. assert that there is an urgent need for further documentation regarding invasive species in the Caspian Sea, since these populations appear to be expanding at an "alarming rate".

The Caspian Sea is afflicted with significant water pollution, much of the source being influx from the Volga River, containing both industrial chemicals as well as eutrophic inducing fertiliser loading.

History

Prehistoric habitation on the shores of the Caspian is documented as early as Paleolithic times; however, much of the research has been directed at human occupation of the Hotu and Belt Caves in the eastern Mazandaran region of northern Persia, based upon Neolithic horizons circa 5000 BC. The early presence of man on the shores of the Caspian was inextricably linked with the Caspian seal.[5]

At the time of Herodotus, the Caspian Sea was considered an outer limit of the known world. Herodotus described the area of the Caspian Sea and the River Araxes as lands beyond the Persians, Medes and Colchians, about which little is known.[6] The Caspian was noted by the Venetian explorer Marco Polo, who gave it the name Abakko Sea. Certain ancient ports on the Caspian such as Abaskun were noted by Ptolemaeus in the second century.[7] Medieval explorers from all directions continued to probe the Caspian region.

The first true scientific expeditions to the Caspian Sea were conducted by the St. Petersburg Academy of Sciences in the period 1768 to 1774. These Russian led explorations included geography, natural history and even sea level fluctuation studies of the Caspian, and extended as far as the Iranian shores. The German scientist Otto Wilhelm Hermann von Abich can, however, be credited with the late nineteenth century geological research that proved to be the basis for modern oil and gas exploitation of the region.